1. Field of the Invention
The present invention relates to an image reading apparatus that has optical image reading functionality, such as a FAX, a copying apparatus, or an MFT, for example. In particular, the present invention relates to an image reading apparatus that improves the influence of dust on a platen glass or a white reference plate when performing reading using an original flow reading method with which an original is scanned that is conveyed at a constant speed by an original conveying unit.
2. Description of the Related Art
Image reading apparatuses are broadly divided, based on their reading method, into apparatuses that adopt a sheet-through method, and apparatuses that adopt a fixed original reading method (moving scanner method). The sheet-through method is a method with which an image reading means provided with a line sensor, for instance, is fixed at an original reading position of a platen glass (original platen), an original carrying unit moves an original in a sub-scanning direction at a predetermined speed, and thereby an image is read. The original carrying unit is also called an ADF (auto document feeder). The fixed original reading method is a method with which an original image is read by moving an image reading means in a state where the original is fixed on a platen glass. In the case of the sheet-through method, if there is dirt at the original reading position on the platen glass, even if this is a dot-like dirt, stripe-shaped noise continuous in the sub-scanning direction occurs on the read image. In view of this, many countermeasures against this have been proposed. For example, Japanese patent No. 3337993 proposes a configuration in which a white sheet facing a reading position is read before reading an original, and if a defective image is detected, the reading position is moved, and if such an image is not detected, original reading is performed using that position as an original reading position. Further, Japanese patent No. 3631135 has proposed a configuration in which a means for detecting dust on an original platen glass is provided, and dust avoidance processing is performed in which detection of dust on the platen glass is implemented by performing idle rotation of a feed belt during a period in which an original is not read, and a flow reading position is changed to a position where there is no dust. Furthermore, Japanese patent No. 3631135 proposes an image reading apparatus that detects the existence of dust on the original platen, forbids a reading operation using the sheet-through method according to the detection result, and allows a fixed original reading operation to be performed.
The configuration disclosed in Japanese patent No. 3631135 in which a belt or a roller is used for the rear side requires higher cost compared to that of the configuration disclosed in Japanese patent No. 3337993 in which a white sheet (white plate) is used. In view of this, a configuration in which a white plate is used is desirable in terms of cost; however, there are some problems with regard to countermeasures if a white plate is used.
A first problem is that soiling of a white plate appears as noise of image data, and it is difficult to remove this noise. In the configuration disclosed in Japanese patent No. 3631135, dust detection is implemented by performing idle rotation of the belt that is facing an original reading unit, and thus it is possible to distinguish between dust on the platen glass and dust on the belt. However, with the invention disclosed in Japanese patent No. 3337993 in which a white sheet is used as the rear side, it is not possible to distinguish between dust on the platen glass and dust on the white plate. With the sheet-through method, a white roller or a white plate arranged at the reading position on the platen glass is read before performing reading and between reading one original and another original, and a shading correction coefficient is determined using the white roller or white plate as a reference. Then, shading correction is performed with respect to image data of the read original. Shading correction is performed in order to correct variations of sensor output (luminance) in the main scanning direction due to the light quantity distribution of a light source, for instance, assuming that the white plate or white roller used as a reference is uniformly white. Accordingly, if the white plate or white roller used as a reference is soiled with dust or the like, a correction coefficient for correcting change in the luminance at the dust position is created. If shading correction is performed on image data obtained by reading an original image using the correction coefficient, a pixel corresponding to the position of the soiling on the main scanning line of the image appears on the original image as linear noise that extends in the sub-scanning direction. In the case of using a white roller, if the dust is very small, by reading an image while rotating the white roller, the sensor output at the position where dust adheres and the output for the portion where dust does not adhere are averaged, and therefore the dust is not very conspicuous, and also is not conspicuous as noise of the read image data. However, if a white plate is used, soiling such as dust on the white plate is read as is, and thus it appears as obvious noise.
Further, a second problem is that if the darkness of soiling such as dust is low, and the soiling is close to white, the difference of a reading darkness level relative to the belt or the roller on the back is small, and thus such soiling is difficult to be distinguished. White dust cannot be distinguished as to whether it is a part of an original image or dust, and thus even dust is scanned as if it is a part of the original image, and thereby a white stripe occurs on the read image. Further, since it is difficult to distinguish whether white dust is dust adhering to the platen glass or dust adhering to the white roller or white plate, there is also a problem that it is difficult to select correction processing for reducing or eliminating its influence.